Layers of imaging elements other than the image forming layer are often referred to as auxiliary layers. There are many types of auxiliary layers such as backing layers, subbing layers, antistat layers, overcoat layers, and the like. In most applications these auxiliary layers are crosslinked by addition of a crosslinking agent in order to improve the physical and chemical properties of the dried layer. A typical auxiliary layer application is a backing layer that provides resistance to scratches, abrasions, blocking, and ferrotyping. The latter two refer to the propensity of layers applied onto the support material or imaging element to stick together as a result of adverse humidity, temperature, and pressure conditions which may occur during manufacture and use of the imaging element. Backing layers must provide these and other physical and chemical properties when employed as very thin layers. Typically these layers are less than one micron in thickness. In addition, when backing layers or other auxiliary layers are used on photographic products they must not adversely affect the sensitometric response of the photographic emulsion layer or reduce the transparency of the processed film.
Glassy, hydrophobic polymers are often employed in auxiliary layers for imaging elements because of their desirable chemical and physical properties. These are most often coated from organic solvent-based solutions. Frequently, a hardener or crosslinking agent is added which will react with the functional groups present on the hydrophobic polymer in order to improve physical properties such as abrasion resistance, moisture insensitivity, high temperature blocking resistance, etc. of the dried layer. Alternatively, water-soluble or water-dispersible polymers may be employed with the appropriate crosslinking agent to provide auxiliary layers with the required properties. Because of continuing environmental pressures to reduce or eliminate organic solvent emissions the latter approach has become much preferred.
A variety of crosslinking agents have been described for use in imaging elements, these include; vinyl sulfones, epoxides, aziridines, isocyanates, carbodiimides, amino formaldehyde resins, and others.
Amino formaldehyde resins are the predominant crosslinking agents for thermosetting coatings. The amino resins most commonly used in coatings are derived from the reaction product of melamine, 2,4,6-triamino-1,3,5-triazine and excess formaldehyde (See Wicks, et al, Organic Coatings, Science and Technology Vol. 1: Film Formation, Components, and Appearance, pages 83-103, Wiley (1992)). Coatings combining melamine crosslinking agents with functional polymer resins are employed industrially in such demanding applications as automobile topcoats. Amino formaldehyde resins have also found widespread use in imaging applications. The most popular amino formaldehyde resins used as crosslinking agents are methoxyalkylmelamines represented by formula (1): ##STR1##
wherein, R represents a C.sub.1 to C.sub.4 alkyl group.
U.S. Pat. Nos. 5,096,975, 5,198,499, 5,318,878, and 5,576,163 describe antistatic layers for imaging elements crosslinked with methoxyalkylmelamines. The melamine provides process survivability and improved physical properties to the antistat layer.
U.S. Pat. Nos. 4,963,461 and 4,824,756 describe light-sensitive microcapsules comprising a silver halide core and a shell containing a melamine-formaldehyde or urea-formaldehyde resin and light-sensitive materials prepared using the microcapsules.
U.S. Pat. No. 4,123,278 describes a subbing composition for polyester film support consisting of 25-60 weight % of a chlorine-containing copolymer, 15-40 weight % of a butadiene copolymer, and 2-10 weight % of a water soluble melamine-formaldehyde or hydantoin-formaldehyde resin.
A drawback inherent to all of the above is that the amino formaldehyde crosslinking agents employed in the auxiliary layers generate formaldehyde as a by product of the crosslinking process. Exposure of the imaging layer to the formaldehyde that is emitted from the auxiliary layer during the manufacture and storage of the imaging element may adversely effect the sensitometric response of the imaging layer. Additionally, worker exposure to formaldehyde during the manufacture of such imaging elements is undesirable from a heath and safety standpoint.
It is therefore the objective of the present invention to provide a coating composition allowing the preparation of at least one auxiliary layer of an imaging element which retains the positive attributes associated with melamine crosslinking while eliminating the emission of formaldehyde.